Re: [PATCH v4 3/3] sched: optimize migration by forcing rmb() and updating to be called once

Peter Zijlstra
2015-11-09T13:29:22Z

On Sat, Oct 24, 2015 at 01:16:21AM +0900, byungchul.park@lge.com wrote:
> +++ b/kernel/sched/core.c
> @@ -1264,6 +1264,8 @@ EXPORT_SYMBOL_GPL(set_cpus_allowed_ptr);
>
> void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
> {
> + unsigned int prev_cpu = task_cpu(p);
> +
> #ifdef CONFIG_SCHED_DEBUG
> /*
> * We should never call set_task_cpu() on a blocked task,
> @@ -1289,15 +1291,14 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
> #endif
>
> trace_sched_migrate_task(p, new_cpu);
> + __set_task_cpu(p, new_cpu);
>
> - if (task_cpu(p) != new_cpu) {
> + if (prev_cpu != new_cpu) {
> if (p->sched_class->migrate_task_rq)
> - p->sched_class->migrate_task_rq(p, new_cpu);
> + p->sched_class->migrate_task_rq(p, prev_cpu);
> p->se.nr_migrations++;
> perf_event_task_migrate(p);
> }
> -
> - __set_task_cpu(p, new_cpu);
> }

I don't think this is safe, see the comment in __set_task_cpu(). We want
that to be last.